Epipelagic euphausids (Euphausiacea) and spring mesoscale features in the Gulf of Mexico

The abundance and distribution of 21 species of euphausids collected in surface waters (0-200 m) at 69 stations of the Gulf of Mexico and the Yucatan Shelf was analyzed. Zooplankton samples were taken during the JS8601 cruise on board the RN "Justo Sierra" of the UNAM, in April-May, 1986. During this survey mesoscale hydrographic features such as anticyclonic and cyclonic gyres, and upwelling, were detected in the area. The most abundant species was Stylocheiron carinatum (29.86%), followed by Euphausia tenera (19.97%) and S. suhmi (15.18%). Species richness increased at night: densities were 3.07 times higher then. Apparently, the migratory behaviour, of euphausids buffered the expected effect of the mesoscale features in the local euphausid community. The Bray-Curtis Index revealed three clusters of stations. The first one represented the influence of coastal and shelf waters on the euphausid community. The second group is a mixed oceanic-shelf community, and the third one was related to the Gulf oligotrophic oceanic waters. A fourth group included shallow areas in which no euphausids occurred. The relatively poor anticyclonic eddies did not influence the local euphausid community although low euphausid densities were expected. The relatively richer features, the cyclones, showed higher average densities. Euphausid densities related to the influence of upwelling areas showed density values similar to those recorded in the anticyclones. Two species (S. suhmi and S. carinatum) occurred in all the Gulf of Mexico regardless of the mesoscale environments or the time of day.